Method and apparatus for magnifying computer screen display

ABSTRACT

A screen magnification tool to scale the content of a computer display screen by distinguishing text and non-text elements of the content and applying different scaling methods to the text and non-text elements. Information about the text elements is stored in a database. Other embodiments are also described.

BACKGROUND

Some embodiments of this invention concern the preparation of magnifiedimages for presentation on a computer display, where textual informationpresent in the magnified image has superior contrast and improved lettershapes, and is generally more legible. Other embodiments are alsodescribed.

Computer displays are commonplace, and are used to present a wide rangeof textual and graphical information. The active portion of a display istypically rectangular and substantially planar. An array of coloredspots, or pixels (for “picture elements”), is usually used to presentthe data. A process known as rasterization is performed to convert datato be displayed from its native format into an appropriate array ofpixels to be included on the display. The native format may be, forexample, endpoints of line segments, parameterized curves, and textencoded according to a standard such as ASCII or Unicode. In accordancewith common software engineering practice, the term “pixel” will be usedherein to refer to both the colored spot visible on a display screen andthe representation in computer memory of data which, when operated on bythe appropriate combination of hardware and software, will cause thecolored spot to appear on a display screen.

Pixels on a display can often be displayed in varying color orbrightness, but cannot be displaced from the points of the array Thesepoints are regularly spaced in X and Y directions at an interval knownas the display's resolution. Resolutions are often measured in dots perinch, or DPI. Ordinary display screens generally have a resolutionbetween a few tens and a few hundreds of dots per inch. Screens intendedfor viewing from a greater distance may have pixels that are physicallylarger, but at the intended viewing distance, the larger pixels maysubtend a similar visual angle as the smaller pixels of an ordinaryscreen (when viewed from a shorter distance). Large and small screens,viewed from the intended distance, can therefore provide an overallsimilar impression for the viewer.

Because pixels are at fixed locations on the screen, it is oftenimpossible to rasterize a line or curve in such a way that pixels lyingprecisely on the intended path can be selected for activation. When nopixel lies exactly on the path, one or more of the nearest pixels aregenerally selected for activation during rasterization. The selectedpixel may be set to a different color or brightness to improve theoverall appearance of the rasterized path; this technique is known asanti-aliasing. However, the selection of pixels that are off the truepath results in a rasterized image that contains errors and distortions.Since pixels are usually quite small, the errors are also negligible inmost cases, but they can become important in some circumstances.

Paths with features that are smaller than a single pixel, or complicatedfeatures that must be rasterized and represented by only a few pixels,are especially susceptible to distortion during rasterization. Displayof text data presents particular problems in this regard: characters areoften displayed at a height of only ten or twelve pixels, and manyletters have small features that are very important in helping a readerto distinguish similar shapes. (Consider, for example, the differencesbetween the letters O and Q and the numeral 0.) Various techniques havebeen developed to improve readability of text presented at modest sizes,since the alternate approach of simply making the text larger has thedetriment of reducing the amount of information that can be presentedsimultaneously on a screen.

Currently, most text presented on display screens is drawn using eithera bitmap font or an outline font. A bitmap font is a collection ofcharacters pre-rasterized at a height of a particular number of pixels;the bitmaps are often carefully tuned to be legible at that size.Unfortunately, since screen resolutions and therefore pixel sizes varyover a moderate range, text presented using bitmap characters will alsovary in size. This is unacceptable in many common situations, such as ina “what-you-see-is-what-you-get” (WYSIWYG) text editor. Furthermore,bitmap fonts do not scale smoothly at other than integral magnifications(that is, magnifications by a factor of a whole number such as 2, 3 or4), and even at integral magnifications, the characters often becomedifficult to read.

Outline fonts, by contrast, contain characters described as collectionsof lines and curves. The collections are represented inmeasurement-independent coordinates, so they can be scaled smoothly toany desired size. After scaling, the characters can be rasterized anddisplayed. This method works well for all but very small text sizes, andis in wide use.

Information to be presented on a computer display is often prepared byan application program. Information from a number of differentapplications is commonly presented simultaneously on a single display.In a common arrangement, each application is unaware of the others, andsimply transmits its own display requests to one or more separatesoftware tasks that receive all the requests and manage allocation ofportions of the physical display so that each application can presentsome of its information. In this common arrangement, the applicationsconvert their high-level information into requests to draw on thescreen, but are unconcerned with the conversion of the requests intopixels on the display, while the display manager receives a sequence ofdrawing requests and causes the appropriate pixels to be activated butis unaware of the high-level information that the drawing requestsrepresent.

If it is desired to magnify the image displayed on a screen, as done byconventional screen-magnifier software tools used by the visuallyimpaired, each displayed pixel is magnified and then a portion of themagnified image is re-sampled to obtain new pixel values for display.Unfortunately, this approach magnifies any errors that occurred duringthe rasterization of the original drawing requests, and the particularsusceptibility of text to rasterization errors may cause magnified textto become ugly and even illegible.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention are illustrated by way of example and notby way of limitation in the figures of the accompanying drawings inwhich like references indicate similar elements. It should be noted thatreferences to “an” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean “at leastone.”

FIG. 1 shows an outline of a character, and the same characterrasterized at two different resolutions.

FIG. 2 shows a word printed in outline, anti-aliased bitmap, and bitmapfonts; each word is magnified a number of times to show the effect oferror accumulations.

FIG. 3 shows a conceptual flowchart of the processes involved inproducing a typical computer display.

FIGS. 4 a and 4 b show the effect of an application-providedmagnification function.

FIG. 5 shows a conceptual flowchart of the processes involved inproducing an image for computer display according to an embodiment ofthe invention.

FIG. 6 provides greater detail concerning one of the elements of FIG. 5.

FIG. 7 shows one way that a portion of a screen may be selected formagnification.

FIG. 8 shows a comparison between the results of two conventionalmethods of producing a magnified screen display and an embodiment of theinvention disclosed herein.

DETAILED DESCRIPTION OF DRAWINGS

FIG. 1 shows an outline of an ampersand character 100, and the samecharacter converted to an array of pixels at two different ratios ofcharacter size to pixel size (110, 120). At the lower ratio 110, thecharacter is rasterized into a 12×11 array of pixels; while at thehigher ratio 120, the character is rasterized into a 24×22 array ofpixels. A comparison of the two characters shows that the larger pixels111 (relative to the size of the character) of the lower-resolutionrasterization produce a poorer approximation of the outlined characterthan do the smaller pixels 121 of the higher-resolution rendering. Theerrors between the desired outline and the activated pixels are morepronounced in the lower-resolution version, but even thehigher-resolution pixel map contains areas inside the outline that arenot filled, and areas outside the outline that contain portions ofactivated pixels.

The errors arise because many pixels do not lie entirely inside oroutside the outline. For those pixels, a heuristic is used to determinewhether or not the pixel should be activated. For example, a pixel mightbe activated if at least 50% of the pixel's area lies within theoutline. Alternatively, a pixel might be shaded so that its color,brightness or saturation is proportional to the amount of the pixel'sarea that lies within the outline. Note that most errors cause theboundaries of the displayed character to vary by less than a pixel'sdimension from the true outline. This is true regardless of the size ofthe pixel relative to the size of the character. Thus, rendering errorsmay be reduced by rendering the character at the largest practical ratiobetween the size of the outline and the size of the pixel. However,rendering text at a multiple of the final resolution and then scalingdown to the final resolution (a technique known as super-sampling)requires additional memory and processing time, and may not producecommensurate improvements in legibility as compared with otherheuristics for rendering text. For most purposes, the “largest practicalratio” is simply the final desired pixel resolution.

FIG. 2 shows a word printed at 12 points in an outline font 201 a, ananti-aliased font 202 a, and a bitmap font 203 a. Below each of those,the same word is printed at successively larger magnifications. It isapparent that only the outline font can be magnified smoothly; theanti-aliased and bitmap fonts contain rasterizing errors that, althoughinvisible or inoffensive at the small size, become increasinglypronounced at larger sizes (202 b-e, 203 b-e), and result in impairedlegibility despite the larger size.

FIG. 3 shows a conceptual flowchart of the processes typically involvedin producing an image for presentation on a computer display.Application programs 301 and 302 issue requests 305 and 306 to drawinformation on the screen. In this flowchart, the requests are shown assimple subroutine calls that are eventually received by operating system(OS) 310. In some systems, the requests may be transmitted through aninterprocess communication mechanism to a different application programrunning on the same computer, or over a wired or wireless network to aprogram running on a different computer.

The receiver of the drawing requests usually manages the allocation ofpixels on the screen and prevents one application from disturbinganother application's display, but is unaware of the high-levelinformation that the drawing requests represent. This division offunctions between application program and display manager has proven tobe useful and flexible, but has the disadvantage that there is noconvenient way to work backwards from pixels on the screen to determinewhat high-level information, or even what drawing request, caused thepixel to be activated. Any information about the true path sought to berepresented by a drawing request is lost during rasterization; thepixels no longer contain error information to show how far they are fromthe true path.

In the illustrated system, OS 310 passes the drawing requests to a chainof cooperating software and hardware modules represented by elements 360and 370. Software modules in this chain are often referred to as“drivers,” and so the chain may be called the “video driver chain.” Thesoftware and/or hardware modules rasterize the requests and prepare apixel map for display. Eventually, video hardware 370 produces a signal,such as a VGA signal, that is sent to the display screen 380. The signalcauses the screen to present an approximation of the drawings requestedby the application programs.

FIGS. 4 a and 4 b show the effect of a document magnification functionin a display screen window, such as is often provided by an individualapplication program. In FIG. 4 a, a word processor document is shown atits default “100%” size. FIG. 4 b shows the same document magnified in a“500%” view. However, although the document text is larger, the menutitles and program control icons indicated in area 400 a are notmagnified along with the text. Instead, the titles and icons aredisplayed at the same size (400 b). Thus, while the magnificationfunction that may be provided by an individual application program isuseful for some purposes, it does not help a visually-impaired personwho cannot make out the menu titles and icons. Furthermore, someapplications provide no magnification function whatsoever. An alternatemethod of producing an enlarged display is useful in this regard.

FIG. 5 shows a conceptual flowchart of the processes involved inproducing an image for a computer display according to an embodiment ofthe invention. Application programs 501 and 502 issue requests 505 and506 to draw information on the screen. In the illustrated embodiment,the requests are made by simple subroutine calls that are eventuallyreceived by operating system 510. In other embodiments, the requests maybe transmitted through interprocess communication to a differentapplication program running on the same computer, or over a wired orwireless network to a program running on a different computer.

The requests are passed to a chain of cooperating software and hardwaremodules 520, 550, 570 and 580, which perform operations detailed below.Where the OS is software produced by Microsoft Corporation, the chain ofcooperating modules can be manipulated with functions provided by theDriver Chain Manager (DCM) libraries. When a DCM-enabled module (or setof modules) is inserted into the chain of cooperating modules, it (orthey) will receive function calls including, for example, DrvTextOut():a command to draw standard text on the screen, DrvBitBlt(): a command todraw standard graphics (i.e. non-text elements) on the screen, andDrvCopyBits(): a command to perform standard copies to the screen. Thesefunction calls are “hooks” that a module can use to trigger specialprocessing in response to graphic operations requested by an upper-layerentity (e.g. application programs 501 and 502 or OS 510).

Once inserted into the driver chain, modules implementing an embodimentof the invention can perform the following operations. Data collector520 separates requests to draw text from requests to draw other figuresand stores information about the requests in database 530. Datacollected may include the location, color, transparency, and extent ofthe figure to be drawn. In the case of requests to draw text, datacollector 520 also stores information such as the requested font, size,and orientation, as well as the requested text itself. The text istypically represented by bytes or words in ASCII, Unicode, or anothercommon encoding system, and stored in that format, or in an equivalentencoded form. The data collector may also cause non-text drawingrequests to be rasterized into shadow area 540. The shadow area may beseparate from the pixel map that video hardware 580 converts to a signaland transmits to display screen 590, so that changes to the shadow areaare not immediately reflected on the screen.

Database 530 can be any memory or disk structure in which informationcan be stored for later retrieval. It need not be a full-featuredrelational database with search, statistics, and other advancedcapabilities. The term “database” as used herein simply means a storagearea where data can be stored for future use.

Compositor 550 takes information from database 530, pixels from shadowarea 540, and a set of parameters 555 communicated from a controlapplication 503, and composes a zoom pixel map 560. The parametersinclude magnifications, translations, and other alterations (such ascolor or contrast changes) that may have been requested by thecomputer's user. These parameters control the composition and renderingof the zoom pixel map 560. The zoom pixel map is made available to theremaining modules in the driver chain, graphics software 570 and videohardware 580. The latter modules (570 and 580) represent the videodriver chain before it is augmented by modules 520 and 550 (andassociated elements 530, 540 and 560), which implement an embodiment ofthe invention. Since the zoom pixel map contains the magnified,translated, and otherwise altered image, the signal eventually producedby the video hardware will cause the display screen to present themodified image.

FIG. 6 provides another conceptual flowchart that details the functionsperformed by the compositor. First, this subsystem obtains zoom, pan,and other parameters from sources including an application programdesigned to control the display magnification system (600). The zoom (ormagnification) parameter controls the level of magnification to beapplied to the shadow area before it is sent to the remainder of thevideo output chain. If the magnification is greater than one, then theshadow area will be larger than the space available on the physicaldisplay screen, and so only a portion of the shadow area can bepresented. This portion is called the viewport. To select the viewport,pan coordinates are used. The two-dimensional pan coordinates locate apoint of the shadow area that will be present on the final displayscreen. Often, this point will be one of the four corners of therectangular viewport, but any other point may be used instead. Forexample, the pan coordinates may select the center of the viewport,instead of one of the corners. One intuitive method of permitting theuser to select the viewport is to connect the pan coordinates to themovement of an input device such as a mouse. When the user moves themouse, the viewport is shifted so that the mouse cursor always remainswithin its boundaries.

The zoom and pan parameters are sufficient to allow the compositor toselect a portion of the shadow area (610). This portion is magnified inaccordance with the zoom parameter and re-sampled at the physical screenpixel dimensions (620) to produce a zoom pixel map. The magnificationand re-sampling may incorporate anti-aliasing or other techniques toimprove the clarity of the image. Note that the shadow area does notcontain active pixels corresponding to text drawn by application programrequests, and therefore the magnification does not introduce the sortsof errors that render such text less legible or illegible. Instead, oncethe zoom pixel map is prepared, the text strings stored in the databaseare rasterized or rendered directly into the map in loop 630. For eachstring, an effective font size (taking into account theoriginally-requested font size and the magnification factor) iscalculated (632) and then the text is rendered in the requested font,color, location, and orientation (635). If the requested font is anoutline font, then rendering the text involves scaling the characteroutlines to the effective font size and activating pixels lying withinthe scaled outlines. After this loop completes, the zoom pixel mapcontains an image suitable for display (640). The image combines themagnified viewport with the rasterized text.

According to this embodiment of the invention, all the elements of thedisplayed image are magnified (including items such as command icons),and all text elements are rasterized directly at their final displaysizes, rather than being rasterized at a smaller size and thenmagnified. Therefore, errors in the rendering of text are limited to thesmallest number of screen pixels possible given the combination of font(outline or bitmap), text size, and use of appearance-improvingtechniques such as sub-pixel hinting and font anti-aliasing.Furthermore, another display of a different viewport or magnification ofthe same overall display can be prepared according to an embodiment ofthis invention, and that second display will also have any errors in therendering of text minimized as described above. In this manner, thevisual quality and legibility of text is greatly improved underarbitrary magnifications.

FIG. 7 shows a full-screen display 700 with several applicationsvisible, including calculator 701, web browser 702, electronic mailclient 703, and screen magnification control application 704. Somecomputer systems distinguish between “active” and “inactive”applications; an “active” application is the one with which the user iscurrently interacting. However, both active and inactive applicationsmay produce output to be displayed on the screen. Magnified images 710and 720 show the magnified contents of two different viewports; in 710,an area near the top left of the full-screen display has been selectedand displayed at 2.5 times magnification, while in 720, an area near thelower right of the display has been selected. Note that magnifiedviewports show enlarged versions of all the displayed information, andnot just the information related to the active application.

The screen magnification control application 704 may permit the user toset (or change) display parameters such as the magnification level,display contrast, colors, and cursor shape.

FIG. 8 shows a comparison between the method of one embodiment of thepresent invention and two conventional methods of displaying magnifiedscreen contents. 800 shows the full screen display, while 810 shows aportion magnified by simple pixel scaling, and 820 shows the sameportion magnified by pixel scaling with smoothing applied. Neither 810nor 820 incorporate separate text handling, so distortions in textresulting from magnifying errors in the characters rendered at anunmagnified size are apparent. Note particularly the text near the mousecursor. In 810, the large blocks of the magnified pixels are clearlyapparent. In 820, the text is somewhat easier to read because asmoothing algorithm has been applied, but the edges of the letters arestill uneven. Magnified portion 830 shows the same area with textrendered separately at the magnified size, according to an embodiment ofthe method herein disclosed. Note that the letters near the cursor (aswell as other letters visible on the screen) are smooth andwell-proportioned. The improvement in text quality is readily apparent.

An embodiment of the invention may be a machine-readable medium havingstored thereon instructions which cause a processor to performoperations as described above. In other embodiments, the operationsmight be performed by specific hardware components that containhardwired logic. Those operations might alternatively be performed byany combination of programmed computer components and custom hardwarecomponents.

A machine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer), including but not limited to Compact Disc Read-Only Memory(CD-ROMs), Read-Only Memory (ROMs), Random Access Memory (RAM), ErasableProgrammable Read-Only Memory (EPROM), and a transmission over theInternet.

The applications of the present invention have been described largely byreference to specific examples and in terms of particular allocations offunctionality to certain hardware and/or software components. However,those of skill in the art will recognize that magnified displays canalso be produced by software and hardware that distribute the functionsof embodiments of this invention differently than herein described. Suchvariations and implementations are understood to be apprehendedaccording to the following claims.

1. A computer-readable medium containing instructions that, whenexecuted by a computer including a processor, a memory, and a displaydevice, cause the processor to perform operations comprising: receivinga first subroutine call to draw a non-text element on a display device;drawing the non-text element into a shadow area; receiving a secondsubroutine call to draw a text element on the display device; storingthe text element in a storage area; composing a first zoom bit mapcontaining a first portion of the shadow area and a first rendering ofthe text element in the storage area; passing the first zoom bit map toa video driver for outputting on the display device; composing a secondzoom bit map containing a second portion of the shadow area and a secondrendering of the text element in the storage area; and passing thesecond zoom bit map to the video driver for outputting on the displaydevice.
 2. The computer-readable medium of claim 1, containinginstructions that, when executed by the computer, cause the processor toperform further operations comprising selecting a magnification level tocontrol the composing and rendering operations.
 3. The computer-readablemedium of claim 1, containing instructions that, when executed by thecomputer, cause the processor to perform further operations comprisingselecting a two-dimensional coordinate of a point within the firstportion of the shadow area.
 4. The computer-readable medium of claim 1wherein the first and second renderings of the text element are producedby increasing a font size of the text element by a magnification leveland causing the text element to be rasterized at an effective font size.5. A computer-readable medium containing instructions that, whenexecuted by a processor, cause the processor to perform operationscomprising: in response to receiving a drawing command that normallycauses non-text elements to be drawn, drawing the non-text elements intoa shadow area; in response to receiving a drawing command that normallycauses text elements to be drawn, storing the text elements in adatabase; and producing a screen image by rasterizing text elementsstored in the database according to a set of screen parameters andcombining the rasterized text elements with a portion of the shadow areacontaining the non-text elements.
 6. The computer-readable medium ofclaim 5, wherein the screen image is a first screen image produced byrasterizing text elements stored in the database according to a firstset of screen parameters and combining the rasterized text elements witha first portion of the shadow area, the medium containing instructionsthat, when executed by the processor, cause the processor to performfurther operations comprising: producing a second screen image byrasterizing text elements stored in the database according to a secondset of screen parameters and combining the rasterized text elements witha second portion of the shadow area.
 7. The computer-readable medium ofclaim 5 wherein rasterizing text elements comprises scaling a characterof an outline font to a calculated size and activating pixels accordingto a location of the pixels relative to the scaled character.
 8. Thecomputer-readable medium of claim 5 wherein the set of screen parametersincludes a magnification level, the computer-readable medium containinginstructions that, when executed by the processor, cause the processorto perform further operations comprising altering a magnification levelin the set of screen parameters.
 9. The computer-readable medium ofclaim 5 wherein the set of screen parameters includes a pan origin, thecomputer-readable medium containing instructions that, when executed bythe processor, cause the processor to perform further operationscomprising altering a pan origin in the set of screen parameters. 10.The computer-readable medium of claim 9 wherein altering the pan originis performed in response to an input of a user of the processor.
 11. Amethod comprising: receiving through a first subroutine call a firstplurality of parameters describing a first desired modification of adisplay presented by a screen device, where the first desiredmodification does not include rendering at least one character; storinginformation about the first desired modification in a first memorylocation; receiving through a second subroutine call a second pluralityof parameters describing a second desired modification of the displaypresented by the screen device, where the second desired modification isrendering at least one character; storing the at least one character ina second memory location; producing an image according to a set ofdisplay parameters, the image composed of at least a portion of theinformation in the first memory location and a pixel map created byrasterizing the at least one character in the second memory locationaccording to the set of display parameters; and displaying the image onthe screen device.
 12. The method of claim 11, further comprising:altering the set of display parameters; repeating the renderingoperation according to the altered set of display parameters; andrepeating the displaying operation.
 13. An article of manufacturecomprising: a screen magnification tool to scale an entire content beingdisplayed on a computer screen by distinguishing text and non-textelements in the content and applying different scaling methods to saidtext and non-text elements so that the text elements are displayed inaccordance with information stored in a database, wherein the text andnon-text elements are displayed at a same magnification level.
 14. Thearticle of manufacture of claim 13, wherein said text elements comprisemenu items, icon names, window titles, and text presented byapplications.
 15. The article of manufacture of claim 14, wherein saidtext presented by applications is text presented by inactiveapplications.
 16. The article of manufacture of claim 13, wherein thedifferent scaling methods include re-rendering the text elements tomatch the same magnification level, and magnifying rendered non-textelements from a prior magnification level to match the samemagnification level.
 17. An article of manufacture comprising: a screenmagnification tool to scale an entire content being displayed on acomputer screen by distinguishing text and non-text elements in thecontent and applying different scaling methods to said text and non-textelements so that the text elements are displayed in accordance withinformation stored in a database, wherein the different scaling methodsinclude re-rendering the text elements but not the non-text elements toscale the entire content.
 18. The article of manufacture of claim 17,wherein the different scaling methods include magnifying renderednon-text elements from a prior magnification level.
 19. The article ofmanufacture of claim 17, wherein said text elements comprise menu items,icon names, window titles, and text presented by applications.
 20. Thearticle of manufacture of claim 19, wherein said text presented byapplications is text presented by inactive applications.
 21. A systemcomprising: a processor, the processor to receive subroutine calls todraw a non-text element on a display device, to draw the non-textelement into a shadow area, and to draw a text element on the displaydevice; a memory, the memory to store the text element; and wherein theprocessor is to generate a first zoom bit map containing a first portionof the shadow area and a first rendering of the text element in thememory, pass the first zoom bit map for output on the display device,generate a second zoom bit map containing a second portion of the shadowarea and a second rendering of the text element in the memory, and passthe second zoom bit map for output on the display device.
 22. The systemof claim 21, wherein the processor is to select a magnification level tocontrol the generating and rendering operations.
 23. The system of claim21, wherein the first and second renderings of the text element areproduced by increasing a font size of the text element by amagnification level and causing the text element to be rasterized at aneffective font size.